Google chat. talk or video. Or a hangout on +. The chat works with other clients like telepathy on linux too. I'm sure there's no shortage of non-google clients on windows or mac. No matter you can just use the browser and do just fine, no special app required.

Depends if you want mainstream appeal. Simplicity and ease of use are important factors. A lot of people have Skype, and even if they don't it's trivial to get and set up for use.

Video chat is definitely gaining popularity as internet speeds increase, even though it can seem gimmicky. It's also used in Skype quite a bit for video-podcasts because there really aren't that many great alternatives, especially free (as in beer). There is often business-oriented conferencing software, but they have limited licenses so it's much more difficult to do things like have special guests.

Mumble has its applications, but it's not really designed for widespread adoption. At least, not yet.

There is an inherent attractiveness to Skype's hybrid-P2P protocol, though unfortunately proprietary. Its dynamically allocated supernodes address a lot of problems including scalability and peer discovery. It will, of course, be slower than a direct client-server architecture, but Skype have admittedly made quite the system. Unfortunately they seem willing to compromise privacy.

Not long after Microsoft bought them, they converted to their products and angered their users with a terrible period of lost mail, lost connections, blackouts, serious security hack-ins (there was a period where one could read anyone else's mail), and other bugs.

Some say they had to quintuple the amount of servers in order to re-settle back to the performance prior to the switch; MS never published numbers.